1 Technical Field of the Invention
The present invention relates generally to a radar such as an FMCW (Frequency Modulated Continuous Wave) radar which is designed to transmit a frequency-modulated radar wave and receive a return thereof from an object through a plurality of antennas to determine the distance to, relative speed, and azimuth or angular direction of the object.
2 Background Art
Recently, radars are tried to be used in an anti-collision device of automotive vehicles. As such as radars, FMCW radars designed to measure both the distance to and relative speed of a target are proposed for ease of miniaturization and reduction in manufacturing cost thereof.
Typical FMCW radars transmit a radar signal Ss, as indicated by a solid line in FIG. 9(a), which is frequency-modulated with a triangular wave to have a frequency increasing and decreasing, i.e., sweeping upward and downward cyclically in a linear fashion and receive a radar return of the transmitted radar signal Ss from a target. The received signal Sr, as indicated by a broken line, usually undergoes a delay of time Tr the radar signal Ss takes to travel from the radar to the target and back, that is, a time lag depending upon the distance to the target and is doppler-shifted in frequency by Fd as a function of the relative speed of the target.
The received signal Sr and the transmitted signal Ss are mixed together by a mixer to produce a beat signal B, as shown in FIG. 9(b), whose frequency is equal to a difference in frequency between the received signal Sr and the transmitted signal Ss. If the frequency of the beat signal B when the frequency of the transmitted signal Ss is increasing or sweeping upward, which will be referred to below as a beat frequency in a modulated frequency-rising range, is defined as fb1, the frequency of the beat signal B when the frequency of the transmitted signal Ss is sweeping downward, which will be referred to below as a beat frequency in a modulated frequency-falling range, is defined as fb2, then the frequency fr due to the time delay Tr and the doppler-shifted frequency fd may be expressed as:                     fr        =                              fb1            +            fb2                    2                                    (        1        )                                fd        =                              fb1            -            fb2                    2                                    (        2        )            
Using the frequencies fr and fd, the distance R to and relative speed V of the target may be expressed as:                     R        =                              c            ·            fr                                              4              ·              fm              ·              Δ                        ⁢                          xe2x80x83                        ⁢            F                                              (        3        )                                V        =                              c            ·            fd                                              2              ·              F                        ⁢                          xe2x80x83                        ⁢            o                                              (        4        )            
where c is the propagation speed of a radio wave, fm is a modulation frequency of the transmitted signal Ss, xcex94F is a variation in frequency (i.e., amplitude) of the transmitted signal Ss, and Fo is a central frequency of the transmitted signal Ss.
The determination of the beat frequencies fb1 and fb2 is made usually using a signal processor. Specifically, the beat signal B is sampled in sequence and subjected to fast Fourier transform (FFT) in each of the modulated frequency-rising and -falling ranges to find a frequency spectrum of the beat signal B. Frequency components showing peaks in signal strength within the modulated frequency-rising and -falling ranges are determined as the beat frequencies fb1 and fb2, respectively.
The sampling frequency fs of the beat signal B, as is well known in the art, needs to be twice an upper frequency limit of the beat signal B. Specifically, the frequency variation A F and a modulation cycle 1/fm of a radar wave are so set that frequency components of the beat signal B due to returns of the radar wave from targets present within a preset target detecting range may fall within a band preset below the upper frequency limit of the beat signal B.
Usually, returns of the radar wave from stationary objects such as footbridges or buildings near a road much bigger in size than ordinary automotive vehicles are strong in level even if they are out of the target detecting range (such objects will be referred to as long range targets below). Therefore, when the radar receives a radar return from the long range target, it will cause the beat signal B to contain, as shown in FIG. 10(a), a frequency component exceeding the upper frequency limit. FIG. 10(a) illustrates a frequency spectrum of the beat signal B. In this case, when the beat signal B is sampled and subjected to the FFT, it will cause the frequency component due to the long range target exceeding the upper frequency limit of the beat signal B to be shifted, as indicated by a broken line, to a location that is symmetric with respect to half the sampling frequency fs, so that it appears as a frequency peak within the preset band. This causes the radar to identify the long range target as lying within the target detecting range in error.
Even in the absence of the long range targets, the FFT of samples of the beat signal B may cause any noise components, as shown in FIG. 10(b), to move from outside the upper frequency limit of the beat signal B to inside the preset band, thereby resulting in rise of a noise floor within the preset band, which leads to a drop in SN ratio, thus resulting in lowering of the radar performance. In order to avoid this problem, an anti-aliasing filter may be coupled to an output of the mixer to remove, as shown in FIG. 10(c), noise components lying outside the preset band, especially frequency components over half the sampling frequency fs from the beat signal B produced by the mixer.
Electronically-scanned radar systems are also known as being designed for spreading the target detecting range or improving the accuracy of determining the angular direction of a target. Such a type of radar system works to receive a return of a radar wave from a target through a plurality of antennas and determine the angular direction of the target based on differences in phase and level of the signals received by the antennas. For instance, U.S. Pat. No. 6,292,129 to Matsugatani et al. (corresponding to Japanese Patent First Publication No. 2000-284047), assigned to the same assignee as that of this application, teaches the electronically-scanned radar system which uses a single mixer for decreasing manufacturing costs. The mixer is designed to process signals received by a plurality of antennas in time division to produce a beat signal. In the following discussion, combinations of transmit antennas and receive antennas will be referred to as channels, respectively.
The use of an anti-aliasing filter in the above system in which the mixer processes inputs from the antennas in time division gives rise to a difficulty in deriving information about targets accurately. Specifically, if a cycle in which the channels are switched from one to another is defined as 1/fx, a time division-multiplexed signal inputted to the mixer contains a harmonic that is an integral multiple of a frequency fx. An output of the mixer, that is, the beat signal B, thus, contains a frequency component arising from that harmonic, resulting in an increase in frequency band of the beat signal B. This causes the anti-aliasing filter to eliminate information as well which is required to demultiplex the time division-multiplexed signal into discrete components for the respective channels. This results in overlapping of the discrete components, thus leading to a difficulty in sampling the levels of the signals received by the antennas accurately.
Radar systems using a plurality of transmit antennas work to multiplex a radar wave in time division. Therefore, even if mixers are provided one for each receive antenna, they receive returns of time division-multiplexed components of the radar wave, respectively, thus giving rise to the same problem as described above.
The determination of an angular direction of a target as functions of differences in phase and intensity of return signals received by a plurality of channels requires time consistency or synchronization between the return signals. To this end, all data items required for the FFT in each channel need to be acquired within a sweep time T (=1/2fm) required for each of upward and downward sweeps of the frequency of a transmit signal (i.e., within each of the modulated frequency-rising and -falling ranges).
If a total number of data items to be sampled within the sweep time T is defined as n (=number Nc of channelsxc3x97number Dpc of data items to be sampled in each channel), and a time interval at which the channels are switched from one to another is defined as 1/fx, the sweep time T may be expressed as:                     T        =                              1                          2              ·              fm                                =                      n            fx                                              (        5        )            
where the sampling frequency fs in each channel=fx/Nc. Note that the channel switching interval 1/fx is restricted to less than a switching speed of a high-frequency switch working to switch between connections of antennas with a mixer or an operation speed (i.e., a sampling speed) of an A/D converter working to sample the beat signal, whichever is the slower.
If the number Dpc of data items to be sampled in each channel is fixed, the sweep time T depends only on the number Nc of channels. Increasing the number Nc of channels in order to improve the ability of determining the angular direction of a target, thus, results in an increase in sweep time T.
Such an increase in sweep time T in typical FMWC radars, however, results in an undesirable decrease in a radar range capable of determining the relative speed V of a target. Specifically, the beat frequencies fb1 and fb2 that are produced by the FFT are measured at an integral multiple of a unit frequency 1/T that is the reciprocal of the sweep time T. The resolution xcex94f of the beat frequencies fb1 and fb2, i.e., resolutions xcex94fr and xcex94fd of the frequencies fr and fd are, thus, given by equation below.                               Δ          ⁢                      xe2x80x83                    ⁢                      f            ⁡                          (                              =                                                      Δ                    ⁢                                          xe2x80x83                                        ⁢                    fr                                    =                                      Δ                    ⁢                                          xe2x80x83                                        ⁢                    fd                                                              )                                      =                              1            T                    =                      fx            n                                              (        6        )            
The resolution xcex94R in range of the FMCW radar is expressed by equation (7) below which is rewritten by substituting xcex94fr for fr in the right side of equation (3).                               Δ          ⁢                      xe2x80x83                    ⁢          R                =                                                            c                ·                Δ                            ⁢                              xe2x80x83                            ⁢              fr                                                      4                ·                fm                ·                Δ                            ⁢                              xe2x80x83                            ⁢              F                                =                      c                                          2                ·                Δ                            ⁢                              xe2x80x83                            ⁢              F                                                          (        7        )            
The resolution xcex94V in determining the relative speed of a target is expressed in equation (8) below which is obtained by substituting xcex94fd for fd in the right side of equation (4) and rewriting it using equation (6).                               Δ          ⁢                      xe2x80x83                    ⁢          V                =                                                            c                ·                Δ                            ⁢                              xe2x80x83                            ⁢              fd                                                      2                ·                                  xe2x80x83                                ⁢                F                            ⁢                              xe2x80x83                            ⁢              o                                =                      c                                          2                ·                                  xe2x80x83                                ⁢                F                            ⁢                              xe2x80x83                            ⁢                              o                ·                T                                                                        (        8        )            
Eqs. (7) and (8) show that increasing the frequency variation xcex94F results in an increase in the range resolution xcex94R, and increasing the sweep time T results in an increase in speed resolution xcex94V, but in an undesirable decrease in a radar range capable of determining the relative speed V of a target.
FIG. 11(a) shows a variation in sweep time T for different values of the sampling frequency fs in one channel in a modulation mode A (185 kHz) and a modulation mode B (370 kHz) if the frequency variation xcex94F is constant (200 MHz). FIG. 11(b) shows a change in radar range within which the distance R to and the relative speed V of a target can be determined. As is clear from the drawings, increasing of the sweep time Tin the modulation mode A results in a decrease in radar range within which the relative speed V can be determined. Note that a maximum measurable distance to an object is determined when fr=fs/2, and a maximum measurable relative speed of the object is determined when fd=fs/4. The number Dpc of data items to be sampled in each channel is 512.
Particularly, radars having multiple channels made up of a plurality of transmit antennas must be on standby for at least a time a radar wave takes to travel from the radar to a target and back in order to eliminate a problem that after the channels has been switched from one to another, a beat signal arising from a return of a radar wave outputted by a preceding one of the transmit antennas from being sampled in error as arising from a return of a radar wave outputted from the transmit antenna of the channel now selected. Specifically, it is necessary to increase the sweep time Tin order to avoid such a problem.
Typical FMCW radar usually receive a return of a radar wave from a road surface through a beam formed by an antenna, which will be added to a beat signal as an unwanted signal component, thereby disturbing detection of a beat frequency within a frequency band of such a signal component. For instance, a return of a radar wave from a road surface, as shown in FIG. 12, received by a side lobe is usually what has entered to a radar from a close distance less than the range resolution of the radar, so that a frequency component arising from a time lag between transmission of the radar wave and reception of the return thereof. The radar wave travels forward of the vehicle and falls on the road surface at an angle xcex8. The incident angle xcex8 is a function of distance between a portion of the road surface on which the radar wave falls and the vehicle. A component of a road surface-to-vehicle speed in a direction of the incident angle xcex8, as indicated by black arrows in the drawing, that is, an apparent relative speed of a target determined based on the return of the radar wave from the road surface, therefore, varies greatly. This variation results in a variation in Doppler shift of the return of the radar wave received by the radar, thus causing unwanted signal components over a wide range to be added to the beat signal.
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide a radar apparatus with improved reliability which is designed to eliminate an error in detecting a distant target present outside a preset radar detecting range and also minimize an adverse effect of a return of a radar wave from a road surface without sacrificing the ability of the radar to determine the distance to and relative speed of a target.
According to one aspect of the invention, there is provided a radar apparatus which may be installed in an automotive vehicle to detect an object present ahead to determine the distance to, relative speed, and azimuth or angular direction of the object. The radar apparatus comprises: (a) a transmit signal generator generating a transmit signal which is so modulated in frequency as to vary with time cyclically; (b) a transceiver having a plurality of channels each of which is made up of a transmit antenna and a receive antenna, the transceiver outputting the transmit signal in the form of a radar wave and receiving a return of the radar wave from a target through any of the channels; (c) a switching circuit switching one of the channels to another which is to be used in the transceiver; (c) a beat signal generator generating a beat signal; and (e) a signal processor sampling the beat signal produced by the beat signal generator and subjecting samples of the beat signal to a given signal processing operation to produce predetermined information about the target.
The switching circuit performs a first switching control mode and a second switching control mode within each cycle of frequency-modulation of the transmit signal. In the first switching control mode, all the channels are selected in sequence in a cycle. In the second switching control mode, a predetermined number of the channels are selected in a cycle. The beat signal generator mixes a return signal received by one of the channels or return signals received by all of the channels selected by the switching circuit with a local signal that has the same frequency as that of the transmit signal to produce the beat signal. When all of the channels are selected in sequence, the beat signal is produced by multiplexing the return signals in time division.
In the second switching control mode, the part of the channels are, as described above, used, thereby increasing the degree-of-freedom to set the cycle of frequency-modulation of the transmit signal and an inclination of the modulated frequency of the transmit signal. For instance, it is possible to shorten the cycle of frequency-modulation for increasing the inclination of the modulated frequency and a cycle in which the beat signal is sampled by the signal processor in each channel. This results in improved accuracy of detecting a desired target.
In the preferred mode of the invention, the transmit signal generator changes a cycle of frequency-modulation of the transmit signal to a time length required to acquire the samples of the beat signal required in an operation of the signal processor as a function of a number of the channels to be switched by the switching circuit. If the inclination of the modulated frequency of the transmit signal is set smaller in the first switching control mode, it is possible to determine the relative speed of the target at high resolution. If the inclination of the modulated frequency of the transmit signal is set greater in the second switching control mode, it is possible to spread a range in which the relative speed of the target can be determined.
The shortened cycle of frequency-modulation results in a decreased spread of an unwanted signal component arising from reflection of the radar wave from a road surface, thus facilitating measurement of a frequency component of the beat signal due to reflection of the radar wave from a desired target, which results in increased accuracy of finding beat frequencies.
For instance, if Fast Fourier Transforms of samples of the beat signal are calculated, beat frequencies fb1 and fb2 i.e., frequencies fr and fd may be expressed in a unit that is the frequency resolution xcex94f, as expressed by Eq. (6), which corresponds to a unit (i.e., LSB) in binary notation used to express the frequencies fr and fd. In the following discussion, a value as expressed using such a unit will be referred to as a frequency point. As can be seen from the above Eq. (8), the magnitude of frequency, as expressed by the frequency point, increases with a decrease in cycle of frequency-modulation (i.e., the sweep time T).
However, the range resolution xcex94R, as expressed by Eq. (7), determined as a function of the frequency resolution xcex94F is independent of the sweep time T. Thus, if the distance R to a target is constant, a value of R/xcex94R will be constant, so that the frequency point indicative of the distance R (i.e., the frequency fr) will also be constant regardless of the sweep time T. The speed resolution xcex94V, as expressed by Eq. (8), determined as a function of the frequency resolution xcex94F changes as a function of the sweep time T. Thus, even if the relative speed V of a target is constant, the frequency point indicative of the relative speed V becomes great as the sweep time T increases.
The unwanted signal component arising from the reflection of the radar wave from the road surface substantially arises from the Doppler shift only. As the sweep time T is decreased, a band of the unwanted signal component, that is, the frequency fd, as expressed by the frequency point, is decreased. The beat frequencies fb1 and fb2 arising from reflection of a radar wave from a target within the preset radar detecting range are located at an interval away from each other that is equivalent to the frequency fd that is a function of the relative speed V across the frequency fr that is a function of the distance R. As the sweep time T is decreased, the beat frequencies fb1 and fb2, as expressed by the frequency point, move close to the frequency fr. In other words, the frequency fd decreases. Therefore, an increase in the sweep time T, as shown in FIG. 13(a), may cause the frequency point indicative of the unwanted signal component to overlap the frequency point indicative of the beat frequency fb1 arising from the target. Conversely, a decrease in the sweep time T, as shown in FIG. 13(b), results in a decrease in width of the frequency point indicative of the unwanted signal component and a shift in frequency point indicative of the beat frequency fb1 away from the unwanted signal component, thus enabling the frequency point indicative of the unwanted signal component to be shifted away from the frequency points indicative of the beat frequencies fb1 and fb2.
The signal processor may be designed to perform a first operation mode in which an angular direction of the target is determined using a component of the beat signal acquired in the first switching control mode and a second operation mode in which a distance to and a relative speed of the target are determined using a component of the beat signal acquired in the second switching control mode. Specifically, the number of the channels selected in the first switching control mode is smaller than that in the second switching control mode. Therefore, if a switching interval of the channels is constant, it will permit the sampling interval in each of the channels to be shortened, that is, a sampling frequency in each of the channels to be increased. The determination of the distance to and relative speed of a target may be achieved only using data acquired in one of the channels. Decreasing the channels to be used, therefore, does not produce an adverse effect on the determination of the distance to and relative speed of the target.
Accordingly, decreasing the channels to be selected in the second switching control mode so that half the sampling frequency in each of the channels may be higher than a maximum frequency of a signal component produced by a return of the radar wave from a long range target present outside a preset radar detecting range prevents that signal component from being shifted by the operation of the signal processor to inside a preset band of the beat signal. This eliminates an error in identifying the long range target as a target present within the preset radar detecting range.
Further, in the second switching control mode, noise components caused by shifts in component of the beat signal produced by the operation of the signal processor such as Fourier transform to within the preset band of the beat signal are smaller than those in the first switching control mode, thus resulting in a reduction in rise in noise floor, which results in improved ability of the radar apparatus to measure the beat frequencies.
The signal processor may work in the first operation mode to estimate a frequency component, which is to be concluded in the beat signal in the second operation mode, using on a frequency component of the beat signal derived by a given operation performed in the first operation mode. The signal processor may perform a digital beam forming operation on the estimated frequency component to determine the angular direction of the target.
When all the channels are used in the first switching control mode, it may cause a frequency component of the beat signal arising from a long range target present outside the radar detecting range to be shifted as a noise to inside the preset band of the beat signal. The frequency component arising from a target present within the radar detecting range may, however, be known in the second switching control mode, thus allowing the angular direction of the target to be determined accurately in the first switching control mode using the known frequency component without removing the noise from the preset band of the beat signal if the noise is inconsistent with the known frequency component.
The signal processor may form, in each of the first and second switching control modes, frequency pairs each of which is made up of a frequency component of the beat signal acquired within a modulated frequency-rising range in which the frequency of the transmit signal rises and a frequency component of the beat signal acquired in a modulated frequency-falling range in which the frequency of the transmit signal falls and determines a distance to and a relative speed of an object using each of the frequency pairs. The processor identifies one of the frequency pairs formed in the first switching control mode which is identical in the distance and the relative speed with any of the frequency pairs formed in the second switching control mode as the target to be acquired by the radar apparatus. Specifically, the radar apparatus detects the same object using two measurements in the first and second switching control modes and identifies it as a target only when the two measurements are identical with each other, thus resulting in improved accuracy of acquiring a target.
Typical FMCW radars are designed to determine the distance R to and relative speed V of a target using Eqs. (3) and (4) on condition that when the frequency of the transmit signal Ss, as shown in FIG. 9(a), sweeps upward, the frequency of the transmit signal Ss is higher than the received signal Sr in the modulated frequency-rising range, while when it sweeps downward, the frequency of the received signal Sr is conversely higher than the transmit signal Ss. However, if there is an object within a short range which shows a higher relative speed, it may cause the received signal Sr to be higher in frequency than the transmit signal Ss in the modulated frequency-rising range or the transmit signal Ss to be higher in frequency than the received signal Sr in the modulated frequency-falling range. The possibility of such an event will increase as the inclination of the modulated frequency of the transit signal Ss increases.
Therefore, if a relation in frequency level between the transmit signal Ss and the received signal Sr has been reversed in either of the modulated frequency-rising and -falling ranges, use of them in Eqs. (3) and (4) will cause an error in determining the distance R to and the relative speed V of a target because the frequencies fr and fd are calculated as absolute values. This gives rise to a problem that even though there are the frequency pairs matched between the first and second switching control modes, they are not identified as a target because the measurements derived in the first and second operation modes are inconsistent with each other.
In order to alleviate such a problem, if there is one of the frequency pairs produced in the first switching control mode which is inconsistent in the distance and the relative speed with any of the frequency pairs produced in the second switching control mode, the signal processor defines the lower of frequencies of the frequency pair inconsistent in the distance and the relative speed as a negative value and calculates the distance and the relative speed again using the negative value.
Specifically, if the inclination of the modulated frequency of the transmit signal in the second switching control mode is, as shown in FIG. 14(b), greater than in the first switching control mode, as shown in FIG. 14(a), the possibility that the relation in frequency level between the transmit signal and the received signal has been reversed in either of the modulated frequency-rising and -falling ranges is lower. The above re-calculation results in improved accuracy of identifying a target when the relation in frequency level between the transmit signal and the received signal is reversed only in the first switching control mode.
The FFT of samples of the beat signal, as shown in FIG. 15, will cause the frequency component fib higher than the sampling frequency fs to be shifted across half the sampling frequency fs (i.e., fs/2) and measured as fbxe2x80x2.
Therefore, if there is one of the frequency pairs derived in the first operation mode which is inconsistent with any of the frequency pairs derived in the second operation mode, the signal processor defines either or both of frequency components of the one of the frequency pairs as being arising from the above FFT-caused shift, determines the distance and the relative speed again, and compares them with those calculated in the second operation mode for identifying the target. Note that the frequency component fb is given by fsxe2x88x92fbxe2x80x2.
The transceiver may include a plurality of receive antennas. In this case, the switching circuit preferably has a switch which is designed to select one of signals received by the receive antennas as the received signal. This allows the beat signal generator to have a single mixer to produce the beat signal.
The radar apparatus may further comprise a failure determining circuit which monitors the presence of an inevitable noise component added to the beat signal produced by the beat signal processor. If such a noise component is not detected, the failure determining circuit provides a signal indicative of occurrence of the failure of the radar apparatus.
In a case where the transmit signal generator includes an oscillator which produces the frequency-modulated transmit signal, an FM-AM conversion noise which arises a variation in power (i.e., amplitude) of the oscillator usually produced as a function of an oscillation frequency is added to the beat signal as the inevitable noise component, so that the transmit signal is modulated in amplitude as well as in frequency. Thus, if there is no such a noise component in the beat signal, it may be determined that the transmit signal generator is malfunctioning.